The present invention relates to fuel cells, particularly to solid oxide fuel cell (SOFC) stacks, and more particularly to a co-flow or counter flow planar fuel cell stack with an integral, internal manifold and a cell casing holder to separately seal the cell, thus providing improved sealing and gas flow as well as easy manifolding of cell stacks.
Solid oxide fuel cells are one of the most promising technologies for power generation. Like all fuel cells, SOFC cells are composed of two electrodes (anode and cathode) and an electrolyte. Since each single cell has a maximum voltage of about IV only, several cells must be stacked together in a stack to yield high voltages for practical applications. The stacking of the cells needs to address the gas flow distribution in the stack as well. The SOFC design closest to commercialization is the tubular design which can be assembled into larger units without the need of a seal. This sealess design is its biggest engineering advantage. However, the tubular geometry of these fuel cells limits the specific power density to low values because the electrical conduction paths are long, leading to high energy losses from internal resistance heating. For these reasons, other fuel cell constructions are being actively pursed at the present time.
The most common alternative design is a planar arrangement with a cross-flow or radial flow arrangement. These planar fuel cells are constructed from alternating flat single cells, which are trilayer cathode electrolyte anode structures, and bipolar plates, which conduct current from cell-to-cell and provide channels for gas flow. Each individual cell, and the bipolar plate associated with every cell in the stack must be sealed together so that they are gas-tight at each manifold face. In addition the manifolds must be sealed gas-tight to the stack to prevent fuel and oxidant gas cross-leakage. The cross-leakage can compromise cell efficiency and is hazardous due to the possibility of explosion. Sealant materials which have thermal expansion coefficent matching with other components of the stack and with satisfactory durability at operating temperatures are not available at the present time. This presents a serious technological shortcoming for planar solid oxide fuel cells.
Planar fuel cell stacks may also be constructed using a co-flow or counter-flow configuration with internal manifolds, as exemplified by U.S. Pat. No. 4,761,349 issued Aug. 2, 1988; No. 5,227,256 issued Jul. 13, 1993; No. 5,480,738 issued Jan. 2, 1996; and No. 5,549,983 issued Aug. 27, 1996. However, most of these designs were mainly developed for electrolyte-supported cells (thick electrolyte with thin electrodes). Since the electrolyte membranes are impervious, the sealing and the stack design are not as complex as for electrode-supported cells (one thick electrode serving as support and a thin film electrolyte). However, the electrode-supported cells have significantly higher performance than the electrolyte-supported cells because of lower resistance of the thin film electrolyte.
In some of the proposed designs such as the conventional cross-flow configuration, the sealing must be done at the edge and corner, which result in higher risk of leakage due to small seal area and less durable stack. Most of the stack designs proposed for electrolyte-supported cells are not applicable to electrode-supported cells because of the leakage through the porous electrode support.
The present invention provides a solution to the above-mentioned problems of planar fuel cell stacks, by providing a planar stack design with separate cell holders for improved sealing and reduced thermal stress problems. This design is particularly suitable for electrode-supported fuel cells because it promote face seal instead of corner seal; however, the design is applicable to electrolyte-supported cells as well. A key feature of the present invention is the cell holder that is separate from the interconnect itself.
It is an object of the present invention to provide an improved fuel cell stack.
A further object of the invention to provide a co-flow planar solid oxide fuel cell stack.
Another object of the invention is to provide a method of constructing a fuel cell stack which can also be used in electrolysis, gas separation, and other electrochemical systems requiring gas-proof separation of gases.
Another object of the invention is to provide a co-flow planar stack with integral, internal manifolding and a casing/holder to separately seal a cell using coventional sealing materials such as ceramic, glass, or glass-ceraminic based sealants.
Another object of the invention is to provide a co-flow planar stack which improves sealing and gas flow, and provides for easy manifolding of cell stacks.
Another object of the invention is to provide a co-flow stack and cell casing/holder which provides improved durability and operation with an additional increase in cell efficiency.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically the invention provides a co-flow planar stack arrangement which can be utilized for solid oxide fuel cells and other electrochemical systems requiring separation of incompatible gases, such as used in electrolysis, gas separation, gas sensors, etc. The present invention overcomes the cross-leakage and other problems associated with prior planar fuel cell stacks designs, by providing a co-flow planar stack with integral, internal manifolding and a cell casing/holder to separately seal each cell using sealants such as materials based on ceramic, glass, or glass-ceramic. Such construction improves sealing and gas flow, and enables easy manifolding of cell stacks. The present invention utilizes a casing/holder containing a cell located intermediate a pair of flow channel/interconnects, with each cell having two pairs of openings at opposite ends which provide the separated co-flow of the fuel and oxidant gases.